In a high data rate TDMA mobile radio communication system, the typical transmission channel experiences multipath interference, in addition to Rayleigh fading. Thus, the radio receiver for a mobile or portable TDMA system must accommodate the reception of multiple replicas of a transmitted signal, each with random magnitude, phase and time delay with respect to the transmitted signal. Without corrective measures, the intersymbol interference (ISI) caused by both multipath and Rayleigh fading will severely degrade the receiver's performance.
It will be appreciated by those skilled in the art that the process of correcting for channel-induced distortion is called equalization. Several types of equalizers may be employed in this effort, such as, for example, linear equalizers, decision feedback equalizers (DFE), and maximum-likelihood sequence estimation (MLSE) equalizers. The remainder of this discussion will concentrate on receivers employing MLSE type equalization.
In 1972 it was theorized by Dr. G. D. Forney Jr., that a maximum-likelihood sequence estimation (MLSE) function was applicable to data transmission system equalization, see G.D. Forney, Jr., "Maximum-Likelihood Sequence Estimation Of Digital Sequences In The Presence Of Intersymbol Interference", IEEE Transactions on Information Theory, Vol. 18, No. 3, May 1972, pp. 363-377. MLSE is a sequence estimation technique which involves maximizing or minimizing an objective function.
Initial attempts at evaluating an objective function utilizing MLSE proved too complex to be of practical use. Therefore, Dr. Forney reformulated the problem and employed the Viterbi algorithm to assist in the computation of the function. It will be appreciated that the Viterbi algorithm significantly reduces the number of computations required to implement MLSE. For a more detailed review of the Viterbi algorithm and MLSE equalization, reference may be made to G. D. Forney Jr., "The Viterbi Algorithm", Proceedings of IEEE Vol. 61, No. 3, March 1973, pp. 268-278.
A receiver structure improving upon the Forney MLSE equalizer was described by Dr. G. Ungerboeck, "Adaptive Maximum-Likelihood Receiver For Carrier-Modulated Data-Transmission Systems", IEEE Transactions on Communications Vol. COM-22, pp. 624-636, May 1974 More recently, another MLSE receiver was disclosed by R. D'avella et al., "An Adaptive MLSE Receiver for TDMA Digital Mobile Radio", IEEE Journal On Selected Areas In Communications. Vol. 7, No. 1, January 1989. In the D'avella system, it is assumed that a carrier frequency offset, caused by either a Doppler induced frequency shift as part of the radio channel characteristic, hereinafter referred to as the channel impulse response (CIR), or a frequency difference between a transmitter and receiver's local oscillator, will cause distortions within each received TDMA signal on a burst-by-burst basis. Consequently, D'avella's system is designed to track and remove such distortion. Utilizing a coherent, decision-directed, detection technique, D'avella's MLSE receiver derives a new estimate of the CIR for each received burst. Next, the matched filter tap gains are computed as the complex conjugate of the estimated CIR samples. Thereafter, a gradient algorithm is used to minimize the mean-square error by adjusting the matched filter's tap gains as disclosed in the previously cited article by G. Ungerboeck. Unfortunately, sequence estimation delays limit the allowable carrier frequency offset ranges that can be compensated for by D'avella's system.
Yet another MLSE type receiver was disclosed in June of 1990 by Kazuhiro Okanoue et al., "An MLSE Receiver with Carrier Frequency Estimator for TDMA Digital Mobile Radio", Proceedings of the Fourth Nordic Conference On Digital Mobile Radio Communication, Oslow Norway, Jun. 26-28, 1990. This MLSE receiver adopts demodulation principles whereby carrier frequency offset estimations are performed as a function of a matched filter's output, before any data signal estimations are made. Accordingly, Kazuhiro Okanoue et al. suggests a non-decision-directed carrier frequency estimation technique. It will be appreciated by those skilled in the art that non-decision-directed detection techniques typically sacrifice performance in favor of speed. It would therefore be extremely advantageous to provide an alternative MLSE receiver arrangement capable of providing carrier frequency offset compensation in a TDMA communication system, while overcoming the shortcomings of the prior art.